Body part recognition inside of blender using a neural net
Consider this to your approach:


feeding your net just a 2d image is limiting (and more work to set it up) when you could feed it whole 3d mesh. You could even build an octree and know what is where in space and feed this to the neural net, basically voxel data instead of pixels
probably the whole neural net thing is not even necessary. There is a popular method of extracting skeletons from meshes by Laplacian smoothing the mesh: Skeleton Extraction by Mesh Contraction:


  
  Our method extracts a 1D skeletal shape by performing geometry contraction using constrained Laplacian smoothing.


You will find this in open source C++ CGAL library here. This is probably enough to know where the neck and crotch is and where limbs end.
other method is: Mean Curvature Skeletons:


  


Code here.